Journal of the Geological Society, London, Vol. 167, 2010, pp. 281–295. doi: 10.1144/0016-76492009-039.

281

Palaeocene–early Eocene inversion of the Phuquoc–Kampot Som Basin: SE Asian

deformation associated with the suturing of Luconia

MICHAEL B. W. FYHN 1,2* , STIG A. S. PEDERSEN 2, LARS O. BOLDREEL 1, LARS H. NIELSEN 2,

PAUL F. GREEN 3, PHAN T. DIEN 4,5, LUONG T. HUYEN 4 & DIRK FREI 2

1Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350,

Copenhagen K, Denmark2Geological Survey of Denmark and Greenland, GEUS, Øster Voldgade 10, DK-1350, Copenhagen K, Denmark

3Geotrack, Melbourne, 37 Melville Road, Brunswick West, Victoria 3055, Australia4Hanoi University of Mining and Geology, Dong Ngac, Tu Liem, Hanoi, Vietnam

5Vietnam Petroleum Institute, Dong Da, Truong Chinh, Hanoi, Vietnam

*Corresponding author (e-mail: mbwf@geus.dk)

Abstract: The little explored Cambodian and Vietnamese Phuquoc–Kampot Som Basin is a Late Jurassic to

Early Cretaceous foreland basin developed in response to the build-up of a palaeo-Pacific magmatic arc. A

combination of seismic data, well data and outcrop geology complemented by fission track and U/Pb analysis

is used to unravel the basin history. This reveals a hitherto unknown earliest Palaeogene basin inversion

associated with the Luconian suturing to SE Asia and the shutdown of palaeo-Pacific subduction underneath

SE Asia. The Phuquoc–Kampot Som Basin and the Khorat Basin in Thailand constitute the erosional

remnants of a larger basin that covered large parts of SE Asia in Late Mesozoic time, and subsequently

became segregated during earliest Palaeogene inversion and erosion. Inversion was focused along the several

hundred kilometres long Kampot and Khmer–Chanthaburi fold belts that confine the Phuquoc–Kampot Som

Basin and merge with the Mae Ping and the Three Pagodas fault zones. These connections, together with local

NW–SE-trending sinistral transpressional faults offshore, indicate a link between initial SE Asian left-lateral

strike-slip faulting and the Luconian suturing. The separation between the once unbroken Khmer–Chanthaburi

Fold Belt and the Phetchabun Fold Belt in Thailand suggests a 50–100 km Cenozoic left-lateral offset across

the Mae Ping Fault Zone.

The Sundaland core of SE Asia arose from the amalgamation of

smaller continental fragments throughout the mid-Phanerozoic

(Fig. 1). The main features of the accretion history are fairly well

described, although the exact timing of continent collision is

debated (Hutchison 1989; Mitchell 1993; Lovatt Smith et al.

1996; Metcalfe 1996, 1998; Stokes et al. 1996; Lepvrier et al.

1997, 2004, 2007; Lacassin et al. 1998; Sone & Metcalfe 2008;

Barber & Crow 2009). The Late Mesozoic to earliest Cenozoic

development is less known, although a series of basins formed

during this period, which may record the regional coeval tectonic

development (Fig. 2). Indeed, the early Palaeogene accretion of

Luconia onto SE Asia has been documented only in Sarawak in

western Borneo despite its proximity to Vietnam (Fig. 1; Benard

et al. 1990; Hutchison 1996; Honza et al. 2000).

Eocene–Oligocene rifting of the Cenozoic basins along the

Vietnamese margin was associated with major left-lateral shear-

ing across narrow fault zones transecting the region (Tapponnier

et al. 1986; Rangin et al. 1995; Fyhn et al. 2009a,b,c). The

activity of these fault zones has generally been linked with the

southeastward displacement of the region caused by the Indian–

Eurasian collision during the mid-Cenozoic (Tapponnier et al.

1986; Lacassin et al. 1993, 1997). However, an earlier onset of

sinistral shearing related to Cretaceous to early Palaeogene

Tethys subduction along the western SE Asia margin or the

accretion of western Myanmar onto SE Asia has been documen-

ted (Morley 2004; Watkinson et al. 2008; Searle & Morley in

press). S-type magmatism and metamorphism along the north-

western rim of Sundaland combined with moderate basin inver-

sion in the northern Khorat Basin suggests that Cretaceous–early

Palaeogene convergence along western SE Asia had a major

impact on the regional tectonic development (Charusiri et al.

1993; Mitchell 1993; Lovatt Smith et al. 1996; Barley et al.

2003; Mitchell et al. 2007; Searle et al. 2007; Barber & Crow

2009).

Knowledge of the Indochinese evolution from after the Late

Palaeozoic–Early Mesozoic accretion of Sundaland until the

onset of Cenozoic deformation associated with the Himalayan

orogeny is fragmentary. Hence, the potential link between

Indochinese basin evolution and Late Mesozoic–earliest Ceno-

zoic plate convergence along the eastern margin of Sundaland,

similar to that described from the western part of Sundaland, is

little explored.

This paper investigates this ‘missing link’ between Late

Mesozoic basin formation and mid-Cenozoic deformation asso-

ciated with the Himalayan orogeny. Based on a study of the Late

Jurassic to Early Cretaceous Phuquoc–Kampong Som Basin, we

present a model for the basin development and inversion of the

Late Mesozoic Indochinese basins. The model includes a

reinterpretation of the late-stage accretion of SE Asia in addition

to the onset age and mechanism of left-lateral shearing within

southern Indochina. The study is based on analysis of c. 18

000 km of multichannel 2D seismic data from offshore south

Vietnam tied to wells (Fig. 3). The seismic analysis was com-

bined with outcrop studies on the SW Vietnamese and Cambo-

dian mainland and on nearby islands. A fully cored, nearly

500 m deep well was drilled through Lower Cretaceous (Aptian)

sediments on Phuquoc Island, complementing the outcrop study.

Exhumation ages and timing of magmatism were assessed using

apatite fission track analysis (AFTA) and U/Pb zircon dating

respectively.

Summary of the regional geology

Accretion of Sundaland

A series of latest Permian–Triassic Indosinian sutures outline

smaller Gondwana-derived continental fragments that constitute

the Sundaland core of the SE Asian ‘promontory’ (Figs 1 and 2;

Fig. 1. Structural outline of the SE Asian

region illustrating the main Cenozoic

structures of the area. A Mesozoic

magmatic arc outlined Sundaland, which

together with the post-Eocene ages of most

ocean basins to the east of it indicates a

significantly different Mesozoic regional

outline.

Fig. 2. Index map of Indochina with

selected structural elements. Two major fold

belts transect the region and connect to the

Mae Ping Fault Zone (MPFZ) and the

Three Pagodas Fault Zone (TPFZ). The four

structural belts outline the boundaries of

large parts of the Late Jurassic–Early

Cretaceous basins. The Khmer–

Chanthaburi Fold Belt has been offset from

the Phetchabun Fold Belt across the Mae

Ping Fault Zone, which suggests 50–

100 km of Cenozoic left-lateral motion

across the fault zone.

M. B. W. FYHN ET AL .282

Hutchison 1989; Metcalfe 1996; Sone & Metcalfe 2008). The

Nan and Benton Raub Sutures outline some of the principal

Indosinian suture zones of Sundaland. The sutures are considered

to enter the Gulf of Thailand west of the study area as roughly

north–south-trending lineaments paralleling the trend of the

Cenozoic rifts underlying the sea (Fig. 1).

Following the main amalgamation, an Andean-type margin

was established along the fringe of Sundaland (e.g. Metcalfe

1996). Convergence of the palaeo-Pacific (Panthalassa) along the

east coast of Asia resulted in the creation of a magmatic arc

parallel to the continental margin during the Mesozoic. Remnants

of the eastern magmatic arc are found in the Schwaner moun-

tains of Borneo and as Jurassic–Cretaceous igneous complexes

forming the basement of offshore Tertiary basins between

Borneo, the Malayan Peninsula and south Vietnam (Fig. 2; Katili

1973; Haile et al. 1977; Williams et al. 1988; Hutchison 1989,

1996). The arc can be traced from the offshore basins across

south Vietnam to south China and farther to the NE (Jahn et al.

1976; Areshev et al. 1992; Rangin et al. 1995; Zhou & Li 2000;

Li et al. 2004; Nguyen et al. 2004; Thuy et al. 2004). Cenozoic

extension and translation has dislocated the magmatic belt and

differential block rotation contorted its original shape (Haile et

al. 1977; Williams et al. 1988; Leloup et al. 1995; Fuller et al.

1999). In particular, the deviating trend of the magmatic-arc

system observed in the Schwaner Mountain Belt of Borneo

(roughly WNW–ESE) relative to that farther north (roughly

north–south to NE–SW) probably resulted from a 50–908

counter-clockwise rotation of Borneo since the Cretaceous,

indicated by palaeomagnetic investigations (Fig. 1; Haile et al.

1977; Schmidtke et al. 1990; Fuller et al. 1999). From the Late

Mesozoic until the middle Eocene, the known west palaeo-

Pacific plates drifted to the NW to NNW with respect to Asia

(Engebretson et al. 1985; Koppers et al. 2001; Seton & Muller

2008), which most probably controlled subduction along the

Asian margin. Hence, subduction to the NW to NNW along the

magmatic arc of Borneo is compatible with a large subsequent

counter-clockwise rotation of the island, which produced its

present outline. However, because of the restricted preservation

of Late Mesozoic west Pacific oceanic lithosphere, including

potential intra-oceanic divergent, convergent and translational

plate boundaries, reconstruction of the direction of Mesozoic and

earliest Cenozoic Pacific plate motions is somewhat speculative

(Engebretson et al. 1985; Koppers et al. 2001; Honza & Fujioka

2004; Smith 2007; Seton & Muller 2008).

The Jurassic to earliest Palaeocene ages recorded in I-type arc-

related igneous rocks of SE Indochina indicate that convergence

operated along this part of the margin during this period (Table

1; Areshev et al. 1992; Rangin et al. 1995; Hoa 1996; Tri 1999;

Tinh 1998; Trang 1998; Thang 1999; Nguyen et al. 2004; Thuy

et al. 2004). Subduction beneath the southeastern margin termi-

nated during the Palaeocene to late Eocene and is recorded by

the Sarawak orogeny in western Borneo (Hutchison 1996; Honza

et al. 2000). The orogeny coupled with the halt of subduction in

this part of the region are viewed as a consequence of the

collision of Sundaland and the Luconian Block (Fig. 2; Hutch-

ison 1996).

More recently Hall (2009) and Hall et al. (2009) suggested

cessation of palaeo-Pacific subduction already during the Late

Cretaceous as a result of suturing of the Luconia–Dangerous

Grounds Microcontinent to SE Asia. This interpretation was

based mainly on: (1) the assumption of cessation of arc

magmatism around middle Late Cretaceous time, and (2) the

suggestion of Moss (1998) that the Cretaceous to Eocene

Rajang–Embaluh Group in northern Borneo was not part of an

accretionary prism associated with subduction underneath Bor-

neo, as otherwise suggested (Haile 1968, 1974, 1994; Hutchison

1973, 1989, 1991, 1996, 2005; Katili 1973; Hamilton 1979;

Holloway 1981; Williams et al. 1988, 1989; Benard et al. 1990;

Tan & Lamy 1990; Tongkul 1991; Hazebroek & Tan 1993). In

the central northernmost part of Kalimantan, Moss (1998) found

no evidence for overall northwards-younging, scraped-off tracts

within the Rajang–Embaluh Group, or of strong deformation or

metamorphism of the Rajang–Embaluh Group. Consequently,

the Rajang–Embaluh Group was suggested to consist of sedi-

ments deposited in a remnant ocean basin (Moss 1998). How-

ever, all of the above-mentioned features have been documented

in Sarawak and other parts of Borneo by, for example, Hutchison

(1996), Omang & Barber (1996) and Honza et al. (2000).

Although volcanism seems to have peaked during Cenomanian–

Turonian time, arc volcanism did not terminate prior to the end

of the Cretaceous in Borneo (Hutchison 1996) and in the early

Palaeocene in Vietnam (Table 1; Hoa 1996; Tinh 1998; Thang

1999). An accretionary setting for the Rajang–Embaluh Group

probably existed until that time (Hutchison 1996; Honza et al.

2000). Consequently, we infer that subduction continued along

the Borneo–Vietnam margin until around early Palaeocene time.

Fig. 3. Seismic grid across the study area with available wells and sample

sites. Illustrated seismic lines are marked in bold. AFTA ages and mean

track lengths; A1, 58.8 � 3.8 Ma and 13.93 � 0.18 �m; A2, 89.6 �12.6 Ma and 13.3 � 0.55 �m; A3, 59.1 � 7.9 Ma and 13.26 � 0.32 �m;

B, 51.3 � 3.6 Ma and 13.23 � 0.19; C, 62.7 � 5.9 Ma and 12.94 � 0.49

�m; D, 52.8 � 3.3 Ma and 13.71 �m; E, 53.4 � 2.7 Ma and 13.61 �0.17 �m.

INVERSION OF THE PHUQUOC – KAMPOT SOM BASIN 283

On the opposite eastern side of Sundaland, smaller continental

or arc fragments including the western half of Myanmar accreted

onto SE Asia during Cretaceous to earliest Palaeogene time

(Mitchell 1993; Metcalfe 1996; Barley et al. 2003; Mitchell et

al. 2007; Searle et al. 2007; Barber & Crow 2009; Hall et al.

2009). Morley (2004) and Watkinson et al. (2008) identified

Cretaceous to earliest Palaeogene transpression across western

Sundaland forced by either Tethys subduction or accretion of

western Myanmar onto SE Asia. However, transpression across

the region has more commonly been regarded as a result of the

subsequent India–Eurasia collision (Tapponnier et al. 1986;

Leloup et al. 1995, 2001; Lacassin et al. 1993, 1997).

Late Jurassic–Cretaceous Sundaland basins

A number of Late Jurassic to Cretaceous and earliest Cenozoic

basins are situated across large parts of Sundaland (Koopmans

1968; Gobett & Hutchison 1973; Rishworth 1974; Khoo 1977;

Harbury et al. 1990; Mouret et al. 1993; Heggemann et al. 1994;

Lovatt Smith et al. 1996; Racey et al. 1996; Lovatt Smith &

Stokes 1997; My et al. 2002; Dien et al. 2008). Although the

sediments of these basins are grouped into various locally defined

groups and formations, their remarkably uniform stratigraphy has

led to speculations of a common origin (e.g. Koopmans 1968;

Rishworth 1974; Khoo 1977; Heggemann 1994). Indeed, a single

vast Late Mesozoic basin, subsequently split up by erosion and

Table 1. Compilation of Middle Jurassic to earliest Palaeogene radio-metric ages of south Vietnamese intrusive rocks

Radiometric age (Ma) Igneous complex Reference

183 � 3 VC 1182 VC 1178 � 5 ? 2177 � 2 VC 1166 � 4 VC 1159 � 5 ? 2158 H DC 1157 DQ 1157 � 3 VC (?) 3155 � 4 VC 1155 � 3 J VC 1153 � 4 DQ 1153 DQ 1149 � 5 ? 2146 VC 1144 DQ 1143 � 2 B HK 1141 � 1 VC 1140 DQ 1135 � 4 ? 2134 DQ 1131 � 3 F DQ 1128 � 3 DC 1126 � 3 DC 1121 J VC 1121 DQ 1121 � 5 E DQ 1119 � 4 I VC 1119 � 2 DC 1117 � 3 DC 1117 � 2 DQ 1116 � 5 DQ 1115 I VC 1113 � 8 DQ 1112 � 2 DQ 4112 DQ 1111 � 3 DC 1111 � 11 DQ 1110 � 1 CN 1110 TN 1109 � 5 ? 2108 � 3 ? 2108 � 4 ? 2106 J VC 1106 E DQ 1105 � 5 ? 2104 E DQ 1104 � 2 H DC 1100 � 3 B HK 1100 � 2 DQ 4100 � 2 CN 1100 � 2 DC 199 � 2 DQ 199 � 4 A CN 198 � 3 DC 198 � 1 DC 598 DC 198 CN 197 C DC 197 � 1 I VC 197 � 2 D DC 197 � 3 ? 297 � 3 DC 197 � 9 DQ 196 � 1 CN 496 � 1 CN 496 � 10 DQ 196 � 2 DC 196 � 2 ? 3

(continued)

Table 1. (continued )

Radiometric age (Ma) Igneous complex Reference

95�1 DQ 195 � 1 DC 595 � 1 DC 595 � 1 CN 194 � 1 CN 494 � 2 DQ/CN (?) 394 � 4 CN 192 � 1 DC 491 � 1 DC 489 � 1 DC 487 � 2 DC 686 � 3 CN 184 � 2 C CN 184 � 3 A CN 183 � 3 DC 182 � 3 F DQ 182 � 8 DC 179 � 2 DC 178 � 1 DC 178 � 4 D DC 177 � 3 DC 171 � 1 A CN 171 DQ 170 DQ 170 DQ 169 � 3 G DC 162 � 2 PR 160 � 1 DC 1

VC, Van Can Complex; DC, Deo Ca Complex; DQ, Dinh Quan Complex; HK, HonKhoai Complex; CN, Ca Na Complex; TN, Tay Ninh Complex; PR, Phan RangComplex. 1, Map series 1996–1999 (Hoa 1996; Trang 1998; Thang 1999) (K–Arages measured on monomineralic biotite, hornblende and feldspar); 2, Areshev et

al. (1992) (K–Ar ages measured on monomineralic biotite); 3, Rangin et al. (1995)(K–Ar ages measured on whole-rock samples); 4, Nguyen et al. (2004) (U–Pb ageson zircon and titanite, Pb–Pb ages on zircon, and Rb–Sr ages on biotite, K-feldsparand plagioclase); 5, this study (U–Pb zircon ages); 6, Lasserre et al. (1970) (K–Arbiotite ages). Letter pairs and triples A–J denotes samples collected at or near thesame locality.

M. B. W. FYHN ET AL .284

lateral shearing, has been suggested (Tapponnier et al. 1986;

Mouret et al. 1993; Heggemann et al. 1994).

To the north, the Khorat Basin occupies larger parts of eastern

Thailand and the bordering areas of Laos and Cambodia (Fig. 2).

This basin has been suggested to be a molasse basin associated

with the Indosinian orogeny (Hutchison 1989), a thermal sag

basin following Triassic rifting, a foreland basin, most probably

associated with uplift along the Anamitic Fold Belt and along

Sundaland suture zones (Lovatt Smith et al. 1996), or a combina-

tion of foreland flexuring and thermal collapse (Cooper et al.

1989). The Late Jurassic to Cretaceous formation of the basin

and the regional uniform thickness indicates that one of the two

latter models is the most likely (Heggemann et al. 1994; Lovatt

Smith et al. 1996; Carter & Moss 1999; Carter & Bristow 2003).

The basin comprises the Khorat Group of Late Jurassic to Albian

age and the Late Cretaceous to earliest Palaeogene Maha

Sarakham Formation (Lovatt Smith et al. 1996; Racey et al.

1996; Stokes et al. 1996; Lovatt Smith & Stokes 1997). During

the Late Cretaceous, the Khorat Basin experienced mild inver-

sion and reorganization of the depositional pattern suggested to

be related to the coeval accretion of west Myanmar onto western

Sundaland (Lovatt Smith & Stokes 1997).

Apatite fission track ages around 40–60 Ma (middle Eocene–

Palaeocene) in eastern Thailand and neighbouring Laos

document a widespread exhumation event during the earliest

Cenozoic (Mouret et al. 1993; Lovatt Smith et al. 1996; Upton

1999). Increasingly older parts of the Khorat Group are exposed

toward the west along the Khorat Monocline, which flanks the

Phetchabun Fold Belt to the east. In the central part of the fold

belt the Upper Mesozoic section has been completely removed as

a result of deep erosion. The monocline continues along the

southern margin of the Khorat Basin straddling the Thai–

Cambodian border. South of the monocline in Cambodia the

erosion level is comparable with that in the Phetchabun Fold Belt

(Gustavson Associates 1991). Although Neogene deposits within

the Tongle Sap Lake Basin conceal most of the pre-Tertiary

geology, scattered outcrops of the Khorat Group as well as older

rocks north of the Tongle Sap Lake demonstrate pronounced

post-Early Cretaceous erosion (Tien 1991; Vysotsky et al. 1994).

South of the Tongle Sap Lake, a similar erosional pattern

exists. Here an Upper Jurassic–Lower Cretaceous unit termed

the Phu Quoc Formation in Vietnam and Bokor or Cam Pong

Formation in Cambodia crops out as erosional remnants together

with older sediments and intrusions surrounded by Quaternary

sediments (Gustavson Associates 1991; Tien 1991; Vysotsky et

al. 1994).

Only limited available information exists on the geology of

Cambodia and SW Vietnam, and the Phuquoc–Kampot Som

Basin has been interpreted as a rift basin or as a foreland basin

(Vysotsky et al. 1994; Dien et al. 2008). Geological maps show

the basin flanked to the east by a north–south-trending belt of

patchy Palaeozoic to Triassic sediments, metasediments and

intrusive rocks that again fringe the Jurassic to earliest Palaeo-

gene magmatic arc farther to the east (e.g. Tien 1991). The

Upper Jurassic to Lower Cretaceous succession is most comple-

tely exposed along the Elephant Mountains in Cambodia and on

the Vietnamese Phuquoc Island paralleling the western margin of

the belt. To the west, the Phuquoc–Kampot Som Basin is

bordered by a monocline along the Thailand–Cambodia border,

comparable with the western margin of the Khorat Basin.

In addition to the distribution of Jurassic–Cretaceous deposits

onshore Thailand and Indochina, patches of similar deposits,

buried underneath younger Cenozoic sediments in the Gulf of

Thailand, have been reported from Thailand (Fig. 1) (Pradidtan

& Dook 1992; Morley et al. 2004). These patches may represent

erosional remnants comparable with those observed onshore.

Farther to the south on the Malayan Peninsula, variably

preserved Upper Jurassic to Lower Cretaceous strata form

comparable sedimentary units to the Khorat and the Phuquoc

groups (Koopmans 1968; Gobbett & Hutchison 1973; Rishworth

1974; Khoo 1977; Hutchison 1989). As with the Khorat and the

Phuquoc groups, the Malayan equivalents were deposited in a

mainly non-marine setting and are dominated by alluvial or

fluvial sediments with subordinate volcanic and volcanoclastic

rocks (Hutchison 1989; Harbury et al. 1990). The Malayan

sedimentary accumulations have tentatively been suggested to be

rift fills (Gobbett & Hutchison 1973; Harbury et al. 1990).

NNW–SSE-trending folding and reverse faulting in addition to

moderate tilting in places (Gobbett & Hutchison 1973; Hutchison

1989; Harbury et al. 1990) document a post-depositional tectonic

event, as does the deep erosion comparable with that observed in

Cambodia and on Phuquoc Island. The Mesozoic deposits

continue to the north, offshore from the Malayan Peninsula, and

erosional remnants sporadically underlie the late Eocene to

Recent rift and sag basins of the Gulf of Thailand in Malaysian

waters (Hutchison 1989; Ngah 2000).

The Phuquoc–Kampot Som Basin

The Phuquoc–Kampot Som Basin forms an elongated, more than

500 km long sediment-filled depression extending from south-

western Cambodia in the north to the central southern part of the

Gulf of Thailand (Fig. 2). Geological maps of the region (Tien

1991; Vimuktanandana 1999) together with seismic data suggest

that the basin is as an up to c. 150 km wide belt with the basin

axis located approximately along latitudes 103–1048.

Based on available descriptions and the authors’ investigations

of the Phu Quoc Formation from outcrops on Phuquoc Island

and the Cambodian equivalent, the Bokor or the Cam Pong

Formation, as well as interpretation of c. 500 m of continuous

cores, we suggest that this up to 3–4 km thick unit is assigned to

a common group (Gustavson Associates 1991; Vysotsky et al.

1994; My et al. 2002; Linh 2003; Dien et al. 2008). The outcrops

on Phuquoc Island are extensive and well mapped (Linh 2003).

In addition, the 500 m of continuous core with wire-line logs is a

candidate for the type section for the group and are available for

future studies at the Vietnam Petroleum Institute in Ho Chi Minh

City. The cores were taken from the ENRECA II well, drilled at

the southern tip of Phuquoc Island, which encountered an Aptian

succession.

It is beyond the scope of this paper to define the proposed new

lithostratigraphic group formally following the recommendations

of Salvador (1994). In this study we use the informal term ‘the

Phuquoc group’ to encompass the Phu Quoc and the Bokor–

Cam Pong formations.

The studied Barremian to Aptian basin fill is dominated by

continental deposits consisting of laterally continuous fluvial

cross-bedded sandstones interbedded with subordinate lacustrine

and flood plain mudstones. Shallow-marine sandstones contain-

ing Diplocraterion, Skolithos and Thallassinoides burrows form

a minor part of the succession. The fluvial transport direction on

Phuquoc Island varied considerably, as indicated by the orienta-

tion of foresets. However, the content of rhyolite-dominated

volcanic clasts, generally in the range of c. 10%, suggests the

coeval volcanic arc located to the east as the primary upland area

(Dien et al. 2008).

The Khorat and the Phuquoc basins have been suggested to be

a once continuous large basin, later split by left-lateral strike-slip

INVERSION OF THE PHUQUOC – KAMPOT SOM BASIN 285

movements and/or focused erosion (Tapponnier et al. 1986;

Mouret et al. 1993; Heggemann et al. 1994). Although know-

ledge of the pre-Neogene geology of central Cambodia is

restricted at present, a common Late Mesozoic basin history

seems probable given the striking similarity in age together with

the depositional and erosional style of the sediments in the two

basins.

Offshore seismic data reveal the Upper Jurassic to Lower

Cretaceous Phuquoc group as an up to c. 2 s TWT (two-way

travel time) thick seismic mega-sequence corresponding to a

thickness of c. 3–4 km as indicated by seismic stacking velo-

cities and average acoustic velocities measured in the ENRECA

II well. The thickness of the Phuquoc group is governed mainly

by the amount of erosion along the top-Mesozoic angular

unconformity that caps the group (Fig. 4). In addition, gentle

internal wedging causes a stratigraphic thickening toward the

magmatic arc (Fig. 4). This wedge-like geometry, combined with

the relative position and comparable timing of the basin and the

magmatic arc, suggest that the studied part of the Phuquoc–

Kampot Som Basin constitutes the preserved foredeep of a retro-

arc foreland basin that formed in response to the build-up of the

magmatic arc (e.g. DeCelles & Giles 1996; Naylor & Sinclair

2008, and references therein). The obvious lack of syndeposi-

tional rifting together with the high content of locally sourced

coarse-grained material are in accord with this interpretation.

Basin inversion

Structural style and distribution

Exposed Upper Jurassic to Lower Cretaceous strata cropping out

on Phuquoc Island generally show a 10–208 inclination towards

Fig. 4. Seismic transect illustrating intensified structuring toward the Kampot Fold Belt in the east. The Upper Jurassic–Lower Cretaceous Phuquoc group

is significantly deformed and truncated at the base-Neogene unconformity, whereas the Neogene sequence is virtually unaffected. A slight internal

wedging within the Upper Jurassic–Lower Cretaceous succession indicates stronger subsidence toward the coeval magmatic arc to the east, which suggests

a Late Jurassic–Early Cretaceous foreland basin setting.

M. B. W. FYHN ET AL .286

the west and SW, suggesting a slight post-depositional tectonic

tilt (Linh 2003, and authors’ own observation). The Cambodian

part of the Phuquoc group possesses a comparable tilt, suggest-

ing that the associated tectonic event affected a larger region.

Seismic data reveal distinct contractional faults and folds that

are cut off along the top-Mesozoic angular unconformity (Fig.

4). The structural complexity increases towards a north–south-

trending deformational belt to the east that cross-cuts the entire

study area (Figs 2 and 5). Approximately north–south verging

imbricated thrusts and associated folds deform the pre-Cenozoic

successions, although the intense deformation within the belt

renders detailed seismic mapping difficult (Fig. 4). In general,

increasingly older successions subcrop the Mesozoic–Cenozoic

boundary in the belt towards the east as a result of eastward

intensified structuring. The belt represents the offshore continua-

tion of a more than 100 km broad belt exposed onshore (Tien

1991). The belt will hereafter be referred to as the Kampot Fold

Belt after the south Cambodian city. The Phuquoc group crops

out in the up to c. 1 km high Elephant Mountains and on

Phuquoc Island straddling the western part of the belt. Triassic

and older sediments or metasediments in addition to intrusive

and extrusive rocks crop out in patches along the eastern part of

the belt (Tien 1991; Hoa 1996). The Kampot Fold Belt can be

traced as far north as a few tens of kilometres south of the

Tongle Sap Lake and transects the entire study area offshore

covered by seismic data, and thus extends for more than 600 km

(Fig. 2). Farther to the east the Kampot Fold Belt borders the

Jurassic to earliest Palaeogene magmatic arc, which is occasion-

ally exposed in the southernmost parts of Vietnam and Cambo-

dia.

Fieldwork in the southern onshore part of the Kampot Fold

Belt in Vietnam and on islands to the south of the mainland

confirms the presence of a generally NNW–SSE- to north–

south-trending fold-and-thrust system, although intense weath-

ering and dense vegetation limit the exposures. Deformed Late

Jurassic to earliest Palaeogene acidic igneous rocks along with

associated tuffs and agglomerates crop out between Triassic and

older sedimentary and metasedimentary successions in the east-

ern part of the Kampot Fold Belt. The igneous rock complexes

represent the westernmost part of the SE Vietnamese magmatic

arc, with associated agglomerates deposited in limited piggyback

basins. Farther to the east, comparable intrusive rocks crop out as

Fig. 5. Map of the subcrop pattern at the

top-Mesozoic unconformity that delineates

the southern part of the Phuquoc–Kampot

Som Basin. Zones of intense thrusting and

faulting outline the Kampot and the Khmer

fold belts that confine the outline of the

Phuquoc–Kampot Som Basin erosionally.

Simplified onshore pre-Quaternary outcrops

are indicated, outlining the onshore

continuation of the Phuquoc–Kampot Som

Basin, the Kampot Fold Belt and the SE

Indochina Mesozoic magmatic arc. Late

Eocene–Oligocene rifting reactivated older

contractional crustal fabric and downfaulted

part of the top-Mesozoic unconformity

below conventional seismic resolution.

INVERSION OF THE PHUQUOC – KAMPOT SOM BASIN 287

isolated hills surrounded by Quaternary alluvium and as islands

in the gulf area, constituting the southernmost exposed part of

the magmatic arc of Indochina. These intrusive rocks have been

deformed by north–south- to NNW–SSE-trending thrust faults,

similarly to the Phuquoc group and the eastern part of the

deformational belt.

The Kampot Fold Belt represents a strongly eroded orogenic

belt that stretches more than 600 km from central Cambodia to

the central part of the Gulf of Thailand. South of the available

seismic grid the fold belt appear to continue beneath thick

Tertiary deposits in the Malay Basin (Fig. 5).

Offshore, the top-Mesozoic unconformity is characterized by a

distinct relief across the western flank of the Kampot Fold Belt

as a result of a particularly resistant interval in the Phuquoc

group repeatedly subcropping the unconformity because of

folding and faulting (Fig. 6). A comparable feature has been

noted in Thailand, where the Berriasian–Barremian Phra Wihan

Formation of the Khorat Group commonly caps the crests of

high-lying areas along escarpments as a result of its relative

competence and resistance to erosion compared with other

stratigraphic intervals (Upton 1999). In the Phuquoc–Kampot

Som Basin the offshore relief reappears onshore in the up to

.500 m high mountains of Phuquoc Island and in the even

higher Cambodian Elephant Mountains. In addition, mid- and

late Cenozoic extensional fault movements have contributed to

the relief in varying degrees.

The Phuquoc group thins both to the south and the western

part of the area as a result of erosion, and in places has been

completely removed. Part of the erosion is due to rift-shoulder

uplift associated with mid-Cenozoic extension in the Malay and

the Khmer basins, documented by extensional faulting that

transects the Phuquoc group and terminates at the base of or dies

out within the Neogene post-rift succession (Fig. 7). However,

dramatic thickness variation of the Phuquoc group occurs across

constrictional faults, demonstrating that earlier orogenic uplift

was a dominant factor controlling the distribution and thickness

of the Jurassic–Cretaceous succession in the southern and

western part of the area, as it is also farther to the NE.

Orogenic structuring increases toward the Khmer Basin, along

the central part of which the degree of deformation is compar-

able with that observed across the Kampot Fold Belt farther to

the east. A second NNW–SSE-trending deformational belt (here-

after named the Khmer Fold Belt) thus confines the distribution

of the Phuquoc group to the west (Figs 1 and 5). The belt strikes

along the axis of the Khmer Basin toward the coasts of SE

Thailand, where a boundary comparable with the eastern basin

margin is outlined on geological maps of Thailand and Cambodia

(Vimuktanandana 1985; Tien 1991).

Fault trends and associated deformational styles

Two contractional fault trends dominate in the study area,

trending north–south to NNW–SSE and NW–SE to WNW–

ESE, respectively. North–south- to NNW–SSE-trending thrust

Fig. 6. A distinct unconformity caps the Mesozoic succession and developed in response to basin inversion. The subcrop of a restricted competent

stratigraphic interval has resulted in numerous buried ridges, the topography of which reappears onshore. In Thailand outcrops of the Phra Wihan

Formation (Berriasian–Berremian) frequently show comparable features (Upton 1999), probably forming the stratigraphic equivalent to the competent

interval of the buried shoulders offshore. The topography of the top-Mesozoic unconformity located in the eastern half of the section forms the buried

offshore part of the Elephant Mountains–Phuquoc Island mountain chain that outlines the eastern margin of the Phuquoc–Kampot Som Basin.

Fig. 7. A pronounced rift-shoulder uplift associated with middle or late

Eocene to Oligocene rifting is suggested by the truncation of the pre-

Cenozoic succession towards mid-Cenozoic grabens. Erosion related to

this rift event influenced a greater part of the region, as suggested by

AFTA data.

M. B. W. FYHN ET AL .288

faults dominate the deformational belts and occur in imbricate

fault systems documented offshore by seismic data. This fault

style suggests regional east–west to ENE–WSW compression.

Subordinate NW–SE- to WNW–ESE-trending contractional

faults have been mapped in the offshore region (Fig. 5). Many of

these faults are remarkably steep and in places form prominent

palm structures in combination with faults trending more to

north–south and NNW–SSE, suggestive of sinistral transpression

along the c. NW–SE-trending faults (Fig. 8). Similar trending

faults (an order of magnitude smaller) have been studied onshore

and on islands in the Gulf of Thailand. A sinistral transpressional

component is suggested by these faults in harmony with east–

west to ENE–WSW compression.

Timing of inversion

The flat-lying Quaternary to Recent deposits capping the Phu-

quoc group onshore provide little constraint on the timing of

basin inversion. However, interpretation of offshore seismic data

provides a better age control. Distinct compressional faults and

folds terminate at the angular top-Mesozoic unconformity. These

structures strongly influence the depth of truncation along the

unconformity but leave the stratigraphic Upper Jurassic to Lower

Cretaceous thicknesses unaffected. This demonstrates that the

compressional tectonic event post-dates the Early Cretaceous

(Aptian) and predates the late Eocene–Recent overburden. The

Aptian age of the upper Phuquoc group in Vietnam may be a

conservative estimate of the youngest age of the basin fill, as a

substantial section has been removed by erosion, further decreas-

ing the potential period of inversion.

Apatite fission track analyses were carried out on seven

samples from the Kampot Fold Belt in Vietnam, six from

outcrops and one from a well core, to date the inversion more

accurately using the approach described by Japsen et al. (2007)

(Fig. 3, Table 1). Sampling was carried out to cover various

stratigraphic levels across a wide area, to optimize the age

estimate of the inversion. Igneous rocks studied by AFTA were

dated radiometrically (U/Pb on zircon; analytical method de-

scribed by Frei & Gerdes (2009)) to discriminate between

cooling associated with magma solidification and subsequent

cooling events more probably caused by exhumation.

The quality of the AFTA data is generally very high, reflecting

the excellent apatite yield in most samples. The resulting thermal

history is well defined, and overall is regarded as reliable,

displaying a high level of consistency between the seven samples

of varying lithologies. Fission track ages in all samples are

significantly less than the ages of the host rocks, implying post-

formational annealing, which, in the light of the seismic analysis,

is most probably a result of deeper burial.

Apatite fission track ages in six of the seven samples are

similar, ranging from 51.3 � 3.6 Ma to 62.7 � 5.9 Ma (Table 2)

whereas a single sample of a Triassic sandstone gave an older

age of 89.6 � 12.6 Ma. Mean track lengths are generally between

13 and 14 �m, and track length distributions are broad, with

standard deviations generally around 2 �m and a significant

proportion of tracks with lengths down to c. 10 �m.

Thermal history solutions have been extracted from the AFTA

data in these samples following the procedures outlined by

Japsen et al. (2007). Most importantly, we do not try to constrain

the entire thermal history of the sample. Rather we focus on

deriving estimates of the maximum palaeotemperature in single

samples and the time at which the sample began to cool from the

palaeo-thermal maximum (as this is the factor that largely

governs the AFTA data).

As summarized in Table 2, all samples show consistent

evidence of cooling that began in the interval 62–50 Ma

(Palaeocene–early Eocene), and most samples also show evi-

dence of a subsequent late Eocene to early Miocene cooling

episode. The sample of a Triassic sedimentary unit (sample

453023) that gave the older fission track age also preserves

Fig. 8. (a) Seismic transect across a

positive flower structure, and (b) structural

map showing fault outline that together

with the seismic transect indicates left-

lateral transpression across the NW–SE-

trending main fault. The seismic grid is

shown in (b) with the transect emphasized

in bold.

INVERSION OF THE PHUQUOC – KAMPOT SOM BASIN 289

evidence of earlier cooling that began between 130 and 70 Ma.

This probably records cooling after the emplacement of a Late

Cretaceous granite (U/Pb 95.4 � 0.6 Ma), intruded into the

Triassic succession a few tens of metres away from the sample

site. The granite was sampled at the same outcrop (sample

453027) and the AFTA data show that this sample cooled below

125 8C during the early Palaeogene episode. The difference in

thermal histories between these samples remains unexplained,

but the majority of samples provide highly consistent evidence of

cooling from palaeotemperatures in excess of 100 8C during early

Palaeogene time (between 62 and 50 Ma).

Measured vitrinite reflectance levels (Ro) from 0.59 to 0.63%

in the ENRECA II well at slightly shallower depths than the

AFTA sample (sample 453001) suggest maximum palaeotem-

perature in the range 97–104 8C using the Burnham & Sweeney

(1989) kinetic model, and following the methods of Japsen et al.

(2007). This is broadly consistent with the maximum palaeotem-

perature between 110 and 120 8C derived from AFTA data in the

core sample. This confirms that the sampled Early Cretaceous

sedimentary unit began to cool from its post-depositional maxi-

mum in the early Palaeogene, with cooling beginning between 62

and 50 Ma based on data from all samples (Table 1) taken from

units of various stratigraphic ages across a widespread area. The

cooling episode probably stems from exhumation during the

early Palaeogene. Such an episode is consistent with independent

age constraints of the Phuquoc–Kampot Som Basin inversion

provided by Cretaceous and middle Cenozoic deposits that

bracket the inversion unconformity, as indicated by offshore

seismic data and information from wells.

The second episode of cooling, which began between 35 and

20 Ma (latest Eocene to earliest Miocene), probably reflects a

second phase of increased denudation (Fig. 9). This phase occurred

coevally with regional rifting in the adjacent Khmer and Malay

basins as well as along the east coast of Vietnam. Consequently,

the second cooling event is interpreted as a result of enhanced

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Kampot Som Basin exemplified by the ENRECA II well-core sample

taken at 496.5–496.75 m in the well. Heating occurred during the

Jurassic to Cretaceous burial phase as indicated by apatite fission track

annealing and vitrinite data. Subsequent Palaeocene–early Eocene basin

inversion is indicated by seismic data and AFTA ages. A second cooling

event took place from late Eocene to Oligocene time and was probably

caused by uplift along the flanks of the adjacent rift basins.

M. B. W. FYHN ET AL .290

exhumation of the uplifted basin flanks. Indeed, the truncational

pattern towards the Khmer and the Malay basins indicates signifi-

cant rift-shoulder uplift along the flanks of the main rifts, followed

by Neogene subsidence and deposition (Fig. 7).

Regional correlation and linkage of deformational belts

The Khmer–Chanthaburi–Phetchabun Fold Belt

The western boundary of the Phuquoc–Kampot Som Basin can

be traced from the offshore Khmer Fold Belt in the central part

of the Gulf of Thailand to the Chanthaburi Fold Belt onshore

southeasternmost Thailand. The onshore western boundary of the

Phuquoc–Kampot Som Basin mirrors the eastern equivalent in

terms of topography and overall geological composition. As in

the easternmost part of the Phuquoc–Kampot Som Basin,

remarkable linear ridges, a few hundred metres high, parallel the

deformation belt in the westernmost part of the basin. Seismic

data show that a similar ridge pattern exists offshore in the

westernmost part of the basin, buried below the Cenozoic

sediments.

The Chanthaburi Fold Belt crops out onshore as erosional

remnants of deformed Triassic and older rocks, comparable with

those sub-cropping in the top-Mesozoic unconformity as ob-

served in the offshore Khmer Fold Belt. The hills are surrounded

by Quaternary alluvium, which renders detailed structural map-

ping difficult (Vimuktanandana 1985; Morley 2002). However, a

prominent NNW–SSE-trending fabric is evident in the Chantha-

buri Fold Belt, similar to that of the offshore fold belt. This

suggests that the Khmer Fold Belt forms the offshore continua-

tion of the Chanthaburi Fold Belt, both together constituting the

western erosional boundary of the Phuquoc–Kampot Som Basin,

and that the deformational belt stretches more than 500 km from

southeastern Thailand to the central Gulf of Thailand. The trace

of the belt is lost below Quaternary alluvium near the area where

the Mae Ping Fault Zone is predicted to enter Cambodia.

Sone & Metcalfe (2008) regarded the Chantaburi Fold Belt as

the southward continuation of the Sukhothai Fold Belt of north-

western Thailand, chiefly based on the distribution of Triassic I-

type granites. However, we suggest a correlation with the

Phetchabun Fold Belt farther to the east, which delineates the

western erosional boundary of the Khorat Basin and can be

traced as far south as immediately north of the proposed trace of

the Mae Ping Fault Zone, buried underneath Quaternary allu-

vium. Widespread Triassic granites similar to those of the

Sukhothai Fold Belt have been reported from the Phetchabun

Fold Belt (Beckinsale et al. 1979; Charusiri et al. 1993; Stokes

et al. 1996), and the Late Mesozoic–earliest Cenozoic develop-

ment and configuration of the Khmer–Chanthaburi Fold Belt and

the Phetchabun Fold Belt seems remarkably similar. Both belts

strike in a NNW–SSE to NNE–SSW direction, were exhumed

during the Palaeocene to middle Eocene, and include deformed

Cretaceous sediments associated with east–west compression

(Mouret et al. 1993; Heggemann et al. 1994; Lovatt Smith et al.

1996; Stokes et al. 1996; Upton 1999; Morley 2004; Morley et

al. 2007). Moreover, both belts mark the present western

erosional margins of a probably once connected Khorat–

Phuquoc–Kampong Som Basin.

The Mae Ping Fault Zone

The trace of the Mae Ping Fault Zone is poorly confined in eastern

Thailand and even more so farther to the east in Cambodia. The

most commonly assumed fault path strikes towards the Tongle

Sap Lake and farther to the SE towards the Mekong Delta and the

margin of the South China Sea (e.g. Lacassin et al. 1997; Morley

2002, 2004; Morley et al. 2007; Smith et al. 2007). The

combination of sporadically outcropping Triassic and older rocks

between outcrops of the Phuquoc and Khorat groups resembles

the subcrop pattern mapped along the top-Mesozoic unconformity

in the offshore fold belt areas, although widespread late Neogene

alluvium effectively conceals most of the pre-Tertiary units. This

indicates a comparable erosional setting to that of the adjacent

fold belts, where the Phuquoc and the Khorat groups have been

removed as a result of orogenic uplift along the NW–SE-trending

Mae Ping Fault Zone. This supports the suggestion that there was

originally a united Khorat–Phuquoc–Kampong Som Basin. The

uplift may very well have occurred in response to Palaeocene–

early Eocene left-lateral transpression. Regional sinistral trans-

pression along smaller-scale, similar trending faults within the

study area supports this inference, as does the evidence for early

Palaeogene cooling of Khorat Group sediments sampled from the

southwestern part of the basin (Upton 1999). Indeed, the Kampot

Fold Belt continues as far north as near to the alleged trace of the

Mae Ping Fault Zone, where they may merge. This could indicate

a close relation between structural shortening across the Kampot

Fold Belt and Palaeocene–early Eocene sinistral transpression

across the Mae Ping Fault Zone.

Between c. 50 and 100 km of left-lateral displacement seems

to have occurred across the Mae Ping Fault Zone during the

Cenozoic, as indicated by the offset Phetchabun Fold Belt

relative to the Khmer–Chanthaburi Fold Belt. Although the

Quaternary cover impedes a more accurate estimate, the offset

deformation belt provides one of the most reliable markers with

which to evaluate the total offset across the Mae Ping Fault

Zone. A left-lateral offset of 50–100 km is compatible with

recent estimates of Smith et al. (2007), suggesting a 10–30 km

offset during the Oligocene, and an unconstrained offset prior to

this. Lacassin et al. (1993) interpreted at least 35–45 km of left-

lateral movement throughout the life span of the Mae Ping Fault

Zone based on extrapolation from boudin trails. However, those

workers inferred an offset of c. 160 km following Tapponnier et

al. (1986), based on the offset western granite belt of Thailand.

The Three Pagodas Fault Zone

The NW–SE-trending Three Pagodas Fault Zone transects Thai-

land and splays into loosely defined strands as it approaches the

Gulf of Thailand (Morley 2002). One of the splays has been

interpreted to bend to the SSE and enter the gulf in the area

where the Khmer–Chanthaburi Fold Belt continues onshore or

immediately to the west. This suggests a close connection

between the Khmer Fold Belt and the left-lateral Three Pagodas

Fault Zone, and may indicate a pre-late Eocene transpressional

history of the left-lateral Three Pagodas Fault Zone as argued by

Morley (2004).

Regional orogenic control

Suturing along west Sundaland

Left-lateral faulting across the Mae Ping Fault Zone has

generally been attributed to escape tectonics associated with the

Himalayan orogeny and more recently to the accretion of western

Myanmar onto SE Asia, and along the Klong Marui and the

Rangong faults in Thailand to Cretaceous subduction processes

along the western margin of the SE Asia (Watkinson et al.

2008). Consequently, left-lateral faulting has been regarded as a

INVERSION OF THE PHUQUOC – KAMPOT SOM BASIN 291

largely middle Eocene to Oligocene or Late Cretaceous–

Oligocene phenomenon (Tapponnier et al. 1986; Leloup et al.

1995, 2001; Lacassin et al. 1997; Morley 2004). Likewise,

folding and faulting within Mesozoic rocks has been attributed to

the same plate-scale events along the western margin of Sunda-

land. Mouret et al. (1993) assigned a Palaeocene age to the onset

of a later phase of folding and related it to exhumation along the

Phu Phan Uplift in the Khorat Basin. Based on westward

intensified deformation, the forcing mechanism was sought to the

west and was associated with the accretion of western Myanmar,

or alternatively with the subsequent northward indentation of

India. A comparable interpretation was favoured by Upton

(1999) to explain the regional Palaeocene–Eocene exhumation

of the Khorat Basin and the Phetchabun Fold Belt.

Suturing along east Sundaland; the Luconian orogeny

The Palaeocene to early Eocene Phuquoc–Kampot Som Basin

inversion most probably formed part of the same regional

inversion event noted by Mouret et al. (1993) and Upton (1999).

However, correlation of the Khorat Basin inversion with plate-

scale events to the west may be premature, as comparable

deformation along the Kampot Fold Belt took place to the east

of the then united Khorat–Phuquoc–Kampong Som Basin.

Hence, the deformation was concentrated along well-defined fold

belts outlining more stable crustal blocks, and may not contain

conclusive evidence as to the fundamental mechanism.

The coeval timing of the exhumation of the Khorat Basin and

the Phetchabun Fold Belt farther to the north (Mouret et al.

1993; Upton 1999), in addition to the uplift of central Vietnam

located along the line of the Phu Phan Uplift noted by Carter et

al. (2000), suggests that the event influenced a very large region.

Farther to the south, on the Malaysian Peninsula, evidence of a

comparable basin inversion exists, although its timing and extent

remain poorly constrained (Harbury et al. 1990).

The regional nature of the inversion event could indicate that

it was the result of suturing along the Sundaland margin as

suggested by Mouret et al. (1993). In contrast to the model of

Mouret et al. (1993), the Palaeocene cessation of arc-related

magmatism in Vietnam around the onset of inversion may

provide a clue to the fundamental mechanism of inversion (Table

1). The position and trend of the Kampot Fold Belt relative to

the adjacent magmatic arc supports such a link.

The early Palaeogene cessation of arc-related magmatism in

the region marks the breakdown of the Pacific subduction under-

neath Indochina and western Borneo. At around the same time,

the Luconian Block accreted onto Sundaland as recorded by the

Sarawak orogeny, which is particularly evident in NW Borneo

(Benard et al. 1990; Hutchison 1996), suggesting a direct link to

continental suturing along the eastern margin of Sundaland (Fig.

10). In Borneo, deformation of early Eocene and older sediments

and metamorphic rocks, overlain by less deformed middle or late

Eocene to Oligocene deposits and volcanic rocks suggests an

early to middle Eocene age for the orogeny in Sarawak. This is

compatible with, although slightly diachronous to the age of the

orogeny in Indochina and Thailand. However, Hutchison (1996)

suggested a Palaeocene onset of collision in Borneo, in harmony

with the observations from Vietnam.

The Luconian orogeny has received little attention with respect

to the coeval tectonic development of Sundaland. Instead,

Palaeocene and early Eocene deformation and exhumation of

central Sundaland have been linked to accretion events and

subduction along the western margin of SE Asia. The apparent

linkage of the Kampot and the Khmer fold belts of Luconian

affinity with those of central Thailand suggests a close connec-

tion between the suturing of Luconia and the establishment (or

Fig. 10. Simplified reconstruction of the palaeogeographical outline of SE Asia (a) immediately before the accretion of Luconia to SE Asia and

(b) immediately after accretion. Prior to the Luconian suturing a large epicontinental foreland basin formed in association with the rise of a magmatic arc

behind the subducting palaeo-Pacific Ocean and uplift along the Annam Cordillera (AC) farther north. Basin inversion associated with the Luconian

collision resulted in basin segregation as a result of uplift along well-constrained deformation belts. Compressional folding and faulting dominated along

the roughly north–south-trending fold belts, whereas left-lateral transpression took place along the more NW–SE-trending deformation belts.

M. B. W. FYHN ET AL .292

reactivation) of fold belts, basin inversion, and exhumation in

central Sundaland. Moreover, the direct link between the Kampot

and the Khmer fold belts and the Mae Ping and the Three

Pagodas fault zones, combined with their apparent overlapping

timing, suggests that incipient transpression could have been

forced by Luconian suturing along the opposite, eastern Sunda-

land margin. Indeed, left-lateral transpression along parallel

NW–SE-trending faults in the Kampot and the Khmer fold belts

supports this inference.

Reactivation of crustal structures

Structuring around the Phuquoc–Kampot Som Basin and the

Khorat Basin is concentrated along distinct deformational belts.

The Phetchabun and the Chanthaburi fold belts have been viewed

as Indosinian (Permo-Triassic) structural belts parallel to the Nan

Suture (Helmcke 1986; Sone & Metcalfe 2008). The early

Cenozoic exhumation and intense structuring of the combined

Phetchabun–Khmer Fold Belt are consequently viewed as a

reactivation of the Indosinian fold belt. This demonstrates the

importance of inherited weakened crustal belts flanking more

rigid blocks in the distribution of intra-plate deformation. An

equivalent reactivation history may be suspected for the Kampot

Fold Belt, which strikes almost parallel to the Indosinian suture

zones.

The late Eocene–Oligocene rift system in the Gulf of Thailand

has been suggested to follow zones of weakness (Kornsawan &

Morley 2002; Morley et al. 2004). Extensional reactivation of

earliest Cenozoic contractional faults and the concentration of

rifting along the Khmer Fold Belt document that middle

Cenozoic rifts in the eastern part of the Gulf of Thailand

reactivated Palaeocene–early Eocene structural zones that in turn

follow the trace of Permo-Triassic deformational belts.

Conclusion

The Phuquoc–Kampot Som Basin in southwestern Indochina

forms a Late Jurassic to Early Cretaceous retroarc foreland basin

associated with plate convergence and back-arc magmatism

along the eastern Sundaland margin. At the time of formation,

the Phuquoc–Kampot Som Basin was part of a larger Sundaland

basin that included the Khorat Basin located to the north and

continued south into Malaysian territories. Non-marine deposi-

tion prevailed in the basin, although occasional marine incursions

occurred and up to .4 km of sediments accumulated during the

period.

Basin inversion occurred during the Palaeocene–early Eocene

in response to the Luconian suturing onto SE Asia, which also

resulted in basin splitting. The continental accretion affected a

large part of Sundaland from Laos in the north to Peninsular

Malaya in the south. Along the margin of the Phuquoc–Kampot

Som Basin, thrusting and uplift were concentrated within the

several hundred kilometre long Kampot and Khmer–Chanthaburi

fold belts and sinistral transpression took place across local

NW–SE-trending faults. The two fold belts appear to link up

with the Mae Ping and Three Pagodas fault zones, suggesting a

connection between the onset of left-lateral transpression across

Sundaland, Palaeocene–early Eocene basin inversion, and the

accretion of Luconia onto SE Asia. The separation of the once-

continuous Khmer–Chanthaburi–Phetchabun Fold Belt in Thai-

land provides a reliable offset geological marker, suggesting a

Cenozoic left-lateral offset of 50–100 km across the Mae Ping

Fault Zone.

This study was funded by the University of Copenhagen. Additional

funding was obtained through the Danida-sponsored ENRECA project

and a Geocenter Copenhagen grant. We thank PetroVietnam and Vietnam

Petroleum Institute for providing seismic reflection and well data, and for

permission to publish this paper. The Geological Survey of Denmark and

Greenland (GEUS) is acknowledged for providing facilities for data

interpretation. J. Halskov and L. C. Mai are acknowledged for technical

assistance. The kind suggestions of E. Sheldon and C. Pulvertaft helped

improve an earlier version of the manuscript, and the reviews of C. K.

Morley and M. Tingay helped strengthen argumentation.

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Received 17 March 2009; revised typescript accepted 9 October 2009.

Scientific editing by Alan Collins.

INVERSION OF THE PHUQUOC – KAMPOT SOM BASIN 295